Passive imaging system equipped with a range finder

ABSTRACT

The invention relates to a range finding method for a target by means of a pulsed laser ( 4 ) and a device for detecting pulses backscattered by the target, which comprises a step for emitting pulses with a repetition frequency f. The detection device comprises an array of photodiodes ( 21 ) associated with integrators ( 22′ ). The method comprises steps for roughly then finely locating the target, the rough locating step comprising the following substeps:
         integration of a backscattered pulse, over a time window of duration F and offset by an offset Off between the instant of emission of the pulse and that of the start of said integration,       

       with Off&lt;1/f and F&lt;1/f,         as long as the signal is less than a first threshold, the integration steps are reiterated with a new offset Off incremented by approximately F relative to the preceding offset Off,   on completion of these iterations, the target being roughly located within a distance range corresponding to the time window of duration F and offset by the offset Off designated Off g , the fine locating step is implemented, which comprises the following sub steps:   integration of a backscattered pulse over a time window of duration F′ and offset by an offset Off f  equal to Off g ,   the integration steps are reiterated with a new offset Off f  of the time window incremented by an offset d relative to the preceding offset Off f , with d&lt;F′ and Off g  Off f ≦Off g +F according to the detected signal.

This application claims priority to French Patent Application Number 0807397, entitled, Systeme D'Imagerie Passive Equipe D'un Telemetre, filedDec. 23, 2008.

The field of the invention is that of laser range finding by flight timemeasurement, associated with passive imaging in a wide observationfield.

A range finding device for a target is often associated with a passiveimaging system in a multifunction optronics system.

An example of such a multifunctional optronics system is described inrelation to FIG. 1.

It comprises, for the passive imaging part, an optical configuration forforming images on an array detector 2 located in the focal plane of thisconfiguration. The figure shows the front optic 1 of this configuration.The array detector 2 comprises an array of transducers 21 such asphotodiodes which convert the photons received into electrons inproportion to the received lighting; these electrical charges are thenintegrated by the integrators 22 respectively associated with thetransducers, and proportionally converted into voltage by acharge-voltage converter 23. The detector also comprises a rowaddressing circuit 25, a column addressing circuit 26 and a multiplexer24 that multiplexes all the signals obtained from the converters 23 andgenerates an output video signal containing the luminescence informationfrom the array detector (designated images in the figure) that isamplified and converted by amplifiers and analogue-digital converters 11so it can be subsequently processed. The row addressing circuit 25selects the row that is connected to the multiplexer. The columnaddressing circuit 26 generates the control signals for the multiplexer24. The read circuit also comprises, in certain cases, a read sequencingcircuit 27 which generates the control address sequence for theaddressing circuits in order to ensure that the image is completelyread. The integrators, the read sequencer, the addressing circuits andthe multiplexer are elements of the read circuit of the array detector.The integrators 22 are controlled by a control signal emitted by a clockgenerator 3. In certain cases, the read sequencer 27 makes it possibleto generate the addresses corresponding to a small subarray ofphotodiodes to limit the reading to a single area of interest. Theintegration start and end signals, the commands for the read sequenceror even, directly, the inputs of the addressing circuits are generatedby the clock generator 3.

It comprises, for the range finding part:

a pulsed laser 4 formatted by an optic 41, illuminating the target andfrom which the signal is partly backscattered to a range findingdetector 5,an optic 6 with large pupil area to maximize the amplitude of thereceived signal,the lunge finding detector 5, generally produced with an avalanche diodeof silicon or InGaAs, or even CdHgTe (depending on the wavelength of thelaser used) of high bandwidth and with a multiplication factor highenough to enable very weak signals to be detected; a wide band amplifier51 and a band pass filter 52 are linked to the photodiode,a module 7 for calculating distance based on the detected signal and thetime elapsed between the start of the laser pulse (designated pulse syncin the figure) and the detection of said signal.

The clock generator 3 and the distance calculation module 7 are housedin a digital processing unit 8.

The reception field is greater than the coverage of the illuminatedtarget surface area, to take account of the harmonization defectsbetween the lines of sight of the emitter 4 and of the detector 2.

Among the main drawbacks and constraints limiting the range of theserange finding devices based on flight time measurement, the followingare worthy of mention:

the reception field greater than the real useful field which generates aphotonic noise due to the scene background degrading the wantedsignal/noise ratio and, consequently, the range,the avalanche detectors which exhibit relatively high noise figures(typically 5 for InGaAs and Si),the laser pulses delivering high energies per pulse which are slow,bulky and costly.

An additional drawback can be added to this list: the surface area ofthe reception optic 6 is often limited by the sharing of availablesurface area with the other optics dedicated to the passive imagingfunctions (visible and thermal, laser spot detection to produce theangle error measurement, etc.).

Consequently, there still remains a need for an optronic system handlingrange finding and passive imaging functions that simultaneously givesatisfaction to all the abovementioned requirements, in terms of SNR,range finding range, reduced bulk and cost, reaction time.

More specifically, the subject of the invention is a range findingmethod for a target by means of a pulsed laser and a device fordetecting pulses backscattered by the target, which comprises a step foremitting laser pulses at determined instants and with a repetitionfrequency f. It is mainly characterized in that the detection deviceincluding an array detector which comprises an array of photodiodesassociated with integrators, the method comprises the following steps ofroughly locating then finely locating the target. The rough locatingstep comprises the following substeps:

-   -   integration by the integrators of a backscattered pulse,        received by the photodiodes over a time window of predetermined        duration F and offset by an offset Off between the instant of        emission of the pulse and that of the start of said integration,        with Off<1/f and F<1/f,    -   comparison of said integrated signal with a first predefined        threshold,    -   as long as the signal is less than this threshold, the        integration and comparison steps are reiterated with a new        offset Off of the time window incremented by approximately F        relative to the preceding offset Off,    -   on completion of these iterations, the target being roughly        located within a distance range corresponding to the time window        of duration F and offset by the offset Off designated Off_(g),        the fine locating step is implemented.

It comprises the following substeps:

-   -   integration by the integrators of a backscattered pulse received        by the photodiodes over a time window of predetermined duration        F′ and offset by an offset Off_(f) equal to Off_(g),    -   comparison of said integrated signal with a second predefined        threshold,    -   the integration and comparison steps are reiterated with a new        offset Off_(f) of the time window incremented by an offset d        relative to the preceding offset Off_(f), with d<F′ and        Off_(g)≦Off_(f)≦Off_(g)+F, until the signal exceeds this second        threshold if the signal is less than this threshold on the first        iteration, or until the signal passes below this threshold if        the signal is greater than this threshold on the first        iteration, the target being located on completion of these        iterations.

F and Off are, for example, determined according to a distance rangewithin which the target is likely to be located.

Similarly, d is determined according to the required location accuracy.

It preferably comprises a substep for post-integration of the receivedsignals before the comparison substep.

The thresholds are, for example, determined according to a targetestimated minimum signal.

Another subject of the invention is an imaging optronic system whichcomprises an array detector and a digital processing unit for the imagesobtained from the detector, characterized in that it comprises a devicefor emitting laser pulses and in that the processing unit comprisesmeans for implementing the method as described.

The array detector advantageously comprises an array of photodiodesassociated with integrators and is equipped with array row and columnaddressing circuits.

This system makes it possible:

-   -   to use a single detection assembly (optics and detector) for the        combined range finding and imaging functions,    -   to avoid the use of a wide bandwidth electronic amplification        subsystem generating a very low noise for the range finding        channel,    -   to correctly operate the range finding in conditions of strong        backscatter from the atmosphere close to the holder of the        system,    -   to use, for range finding functions, less costly lasers of        reduced bulk, delivering pulse trains of low energy but high        frequency,    -   to combine high performance passive imaging and range finding        functions through one and the same optic and a single detection        block.

Other features and advantages of the invention will become apparent fromreading the following detailed description, given by way of nonlimitingexample and with reference to the appended drawings in which:

FIG. 1, already described, diagrammatically represents a multifunctionoptronic system according to the state of the art; a first zoom on thearray detector is used to show the array of photodiodes and the readcircuit and a second zoom on an element of this array is used to show aphotodiode and its associated integrator and converter,

FIG. 2 diagrammatically illustrate the rough locating phase for thetarget (FIG. 2 a) and the fine locating phase (FIG. 2 b),

FIG. 3 diagrammatically represents a multifunction optronic systemaccording to the invention,

FIG. 4 represents the maximum range finding distance according to thelaser repetition frequency.

According to the invention, the array detector of the imaging system isalso used for range finding purposes. The distance calculation is nolonger obtained from the measurement of the time elapsed between thedeparture of a pulse and the return of the corresponding backscatteredsignal, but according to the following method.

FIGS. 2 a and 2 b illustrate different steps of the range finding methodaccording to the invention. The latter is based on a first phase ofroughly but rapidly locating the position of a target, followed by afiner locating phase.

The phase for roughly locating the target comprises the following steps:

-   -   Emission of laser pulses with a width of the order of 10 ns for        example, at instants determined by the system. The repetition        frequency f is less than a limit frequency so as to avoid the        appearance of overlaps between the pulses emitted and received.        This limit frequency is therefore calculated according to the        maximum range finding range illustrated in FIG. 4. It is        typically greater than 20 kHz which corresponds to a range of 10        km.    -   Integration into time windows of the determined dimension F, at        a frequency equal to that of the emission frequency; the        following applies: F<1/f. A number of integration windows are        preferably opened in synchronism with the arrival of the        backscattered laser pulses, the latter being post-integrated in        the read circuit. The dimension of the time window F is        determined by the corresponding distance range designated Dis        within which the target could be located; furthermore, this        distance range is positioned from an origin distance O        determined by the offset Off between the instant of emission of        the pulse and that of the start of integration; the following        applies: Off<1/f. Finally, an integration window of determined        duration F and offset Off has a corresponding departure position        O of a distance range Dis. For example, an integration window of        13.4 μs covers a distance range of 2 km and an offset of 20 μs        corresponds to a departure position located at 3 km: thus, this        integration window covers a distance of between 3 and 5 km.        Depending on the position of the echo, the latter coincides or        does not coincide with this distance range. If the target is        located within this range, at 4 km for example, the echo is        integrated and therefore detected, that is to say that the        target is roughly located between 3 and 5 km; it is in fact        post-integrated in the course of the integration windows as        illustrated in the figure. However, if it is situated outside,        for example at 6 km, it is not detected.    -   If the echo is not detected, that is to say if it is less than a        predetermined threshold, the process is reiterated but the        integration window is offset by F (new Off=preceding Off+F) to        cover the next range whose departure position O is then located        at 5 km, that is to say to cover the distance Dis between 5 and        7 km. To be sure that there will be no discontinuity with the        next range, a slight overlap is provided between two consecutive        windows: the window is preferably offset by F-δF, with, for        example, δF being of the order of approximately 1% of F. The        process continues until the target is roughly located.

Once this rough locating is completed, the fine locating phase isimplemented. Now knowing the distance range and its position O, the nextphase aims to specify the position of the target within this range.

The term “rough” window is used to designate the window resulting fromthe rough locating step and offset by an offset designated Off_(g).During the fine locating step, the time window is offset within this“rough” time window by successive offsets Off_(f). These successiveoffsets Off_(f) are such that:

new Off_(f)=preceding Off_(f) +d.

Its duration F′ is equal to F or is possibly less, on the one hand so asnot to integrate photons that originate from an unanalyzed portion ofthe scene, on the other hand to optimize the overall range findingduration. The position of the target is determined by these successiveoffsets. In practice, the echo appears (that is, when it is greater thana predetermined threshold), over a limited number of windows anddisappears immediately when the delay d is greater than the pulsedflight time, that is to say, as and when these windows are offset withinthe “rough” window. The following applies:

Off_(g)≦Off_(f)≦Off_(g) +F.

The locating accuracy is determined by these offsets d: the knowledge ofthe distance is acquired with the accuracy defined by the increment ofthe delay d, or 1.5 m per 10 ns.

A time window F of 6.68 μs has a corresponding range of 1 km, and anoffset d of 1.33 μs has a corresponding advance of 200 m over theterrain; in this example, 5 offsets are sufficient to cover a range of 1km with an accuracy of 200 m.

In the example of FIG. 2, the result of the rough phase is that thetarget is located between 3 and 5 km and a time window of duration F of6.67 μs corresponding to a range of 1 km, and time offsets of 1.33 μscorresponding to distance offsets of 200 m have been chosen. Since thetarget is located at 3.5 km, its echo appears and is detected over thefirst 3 windows then disappears from the 4th window: the target istherefore situated between 3.4 and 3.6 km.

According to the invention, the imaging device and the range findingdevice have a common detection assembly. The range finding detector 5,the associated amplifier and filter 51, 52 and the associated optic 6described in the preamble become unnecessary; the distance calculationmodule 7, as described in the preamble, is replaced by a module forimplementing the iterative method described previously, housed by thedigital processing unit 8′. This common detection assembly provides again in sensitivity by maximization of the size of the optic 1.

To be able to handle a range finding function as described previously,the image processing unit 8′ further comprises conventional imagingmeans described in the preamble, a spot detection processing module 10which receives the detected images in order, by comparison with a giventhreshold, to determine the presence or absence of a laser signalbackscattered by a target (echo) and range finding supervision means 9.The range finding supervision means 9 indicate to the clock generator 3′the rough or fine locating phase as well as the offset Off or Off_(f) tobe applied. The clock generator 3′ emits range finding-specific controlsignals in addition to imaging-specific control signals. These specificsignals are: pulse departure signal intended for the laser 4, signalindicating connection or disconnection of a photodiode 21 to or from itsintegrator 22′ in order to perform the post-integration and produce a“gating” function.

Also for the range finding purposes, the array detector 2 according tothe invention has the following characteristics:

the array of photodiodes (256 rows×320 columns for example) is producedon a semiconductor material whose spectral mask is adapted to passiveimaging and to the wavelengths of the laser illumination. Such materialscan, for example, include:

silicon adapted to detect a laser beam that has a wavelength of between0.4 μm and 1.1 μm,

CdHgTe adapted to detect a laser beam that has a wavelength of between0.4 μm and 15 μm,

InGaAs adapted to detect a laser beam that has a wavelength of between0.4 μm and 2.5 μm,

the integration capacity of each integrator of the read circuit isdimensioned size-wise for the constraints of the passive imaging andeach integrator 22′ has, for the range finding, the necessary resources(transistor arrangements) to open integration time windows that are veryshort (some 10 ns to some 10 μs) in order to handle a “gating” functionand post-integrate the received signals;furthermore, this same read circuit allows for:

accurate control of the delay (or offset) Off or Off_(f) between theemission of the laser pulse and the start of the integration of thecorresponding backscattered signal, via digital control signals emittedby the clock generator 3;

post-integration over a number of integration windows of severalbackscattered pulses thanks to the disconnection/reconnection of thephotodiode 21 from and to the integrator 22′, controlled via digitalcontrol signals emitted by the clock generator 3′; this offers thepossibility of using lasers 4 of low pulse power and operating at highfrequencies;

preferably, a windowing that is random in size and position of thedetection array through the intermediary of a selective addressing ofthe rows and columns of the array, which offers the possibility ofselecting, in active mode, a small subarray of photodiodes. This makesit possible to limit the reading to a single area of interest byincreasing the repetition frequency of the laser pulses up to very highfrequencies, for example greater than 20 kHz. The use of a detectionarray, capable of conventional passive imaging, divides the totalobservation field into a large number of small individual fields, equalto the number of transducers in the array. For a given observationfield, if this individual field is very small, the photonic noisegenerated becomes almost negligible thus making it possible to detectsignals of very low amplitudes, compatible with a function for detectinga remote laser spot. The operation of a small portion of the array (afew tens of pixels) allows frames of reduced sizes to be read quickly(in 50 μs to 100 μs). This small portion of the array nevertheless has asize greater than that of a conventional range finder photodiode: theresult of this is a strong reduction in the harmonization constraintsbetween the range finder's emission and reception channels.

By reading just the small area of interest of the array (for example: 32columns×10 rows), the distance information can be acquired in a few ms.

This spatial windowing offers the following advantages:

a sensitivity gain in laser spot and range finding detection byminimization of the background photonic noise,simplification of the observance of the standards concerning ocularsafety,a sensitivity gain by reduction of the bandwidth, and therefore of thenoise, of the associated electronics.

1. Range finding method for a target by means of a pulsed laser (4) anda device for detecting pulses backscattered by the target, whichcomprises a step for emitting laser pulses at determined instants andwith a repetition frequency f, characterized in that the detectiondevice including an array detector (2) which comprises an array ofphotodiodes (21) associated with integrators (22′), the method comprisesthe following steps of roughly locating then finely locating the target,the rough locating step comprising the following substeps: integrationby the integrators (22′) of a backscattered pulse, received by thephotodiodes (21) over a time window of predetermined duration F andoffset by an offset Off between the instant of emission of the pulse andthat of the start of said integration,with Off<1/f and F<1/f, comparison of said integrated signal with afirst predefined threshold, as long as the signal is less than thisthreshold, the integration and comparison steps are reiterated with anew offset Off of the time window incremented by approximately Frelative to the preceding offset Off, on completion of these iterations,the target being roughly located within a distance range correspondingto the time window of duration F and offset by the offset Off designatedOff_(g), the fine locating step is implemented which comprises thefollowing substeps: integration by the integrators (22′) of abackscattered pulse received by the photodiodes (21) over a time windowof predetermined duration F′ and offset by an offset Off_(f) equal toOff_(g), comparison of said integrated signal with a second predefinedthreshold, the integration and comparison steps are reiterated with anew offset Off_(f) of the time window incremented by an offset drelative to the preceding offset Off_(f), with d<F′ and Off_(g)≦Off_(f)Off_(g)+F, until the signal exceeds this second threshold if the signalis less than this threshold on the first iteration, or until the signalpasses below this threshold if the signal is greater than this thresholdon the first iteration, the target being located on completion of theseiterations.
 2. Range finding method for a target according to thepreceding claim, characterized in that F and Off are determinedaccording to a distance range within which the target is likely to belocated.
 3. Range finding method for a target according to one of thepreceding claims, characterized in that d is determined according to therequired location accuracy.
 4. Range finding method for a targetaccording to one of the preceding claims, characterized in that F′≦F. 5.Range finding method for a target according to one of the precedingclaims, characterized in that it comprises a substep forpost-integration of the pulses received before the comparison substep.6. Range finding method for a target according to one of the precedingclaims, characterized in that the thresholds are determined according toa target estimated minimum signal and the number of anypost-integrations.
 7. Imaging optronic system which comprises an arraydetector (2) and a digital processing unit (8′) for the images obtainedfrom the detector, characterized in that it comprises a device foremitting laser pulses (4) and in that the processing unit (8′) comprisesmeans (10, 9, 3′) for implementing the method according to one of thepreceding claims.
 8. Imaging optronic system according to the precedingclaim, characterized in that the array detector (2) comprises an arrayof photodiodes (21) associated with integrators (22′) and is equippedwith array row (25) and column (26) addressing circuits.